For any arbitrary command:
nohup <cmd> > cmd_log.txt &
<cmd> into a separate process, and
nohup (“no hang up”) ensures the process does not terminate once you log out. This can be useful to, for example, SSH into a remote machine and launch a long running command, without forcing you to keep the session open.
The stdout of the process can be viewed live by tailing the log:
tail -F cmd_log.txt
nohup command will echo the PID of the background process:
$ nohup tar -xvzf huge.tar.gz > cmd_log.txt &  31782 $ nohup: ignoring input and redirecting stderr to stdout